In the prior art, monocrystalline silicon substrates are widely used as the semiconductor substrate. Because of their physical limits, they gradually cease from meeting such requirements as higher operating temperature, improved pressure resistance, and higher frequency, and engineers start using expensive substrates of new materials such as monocrystalline SiC substrates and monocrystalline GaN substrates. For example, when power converters such as inverters and AC/DC converters are constructed from semiconductor devices using silicon carbide (SiC) which is a semiconductor material having a wider forbidden band than silicon (Si), they achieve a reduction of power loss which is not reachable by semiconductor devices using silicon. Using semiconductor devices based on SiC, not only the loss associated with power conversion is made smaller than in the prior art, but also the modification of devices toward lighter weight, compact size and higher reliability is promoted.
Such monocrystalline SiC substrates are generally prepared by the method (improved Rayleigh method) involving letting high-purity SiC powder sublimate at a high temperature of 2,000° C. or higher and re-growing SiC on a spaced-apart seed crystal. However, since the steps involved in this preparation method are quite complex and under very rigorous conditions, the resulting substrates are inevitably reduced in quality and yield, that is, the substrates become of very high cost, which prohibits their practical application and utilization in a wide range.
Meanwhile, the thickness of a layer (active layer) which develops a device function actually on these substrates is in a range of 0.5 to 100 μm in any of the above-mentioned applications, whereas the remaining thickness portion is a portion that mainly plays the role of mechanical support for handling, that is, a so-called handle member (substrate) which is not limited in defect density or the like.
Thus, a recent study is made on substrates of the structure that a relatively thin monocrystalline SiC layer having the minimum thickness is bonded to a polycrystalline SiC substrate via a layer of a ceramic such as SiO2, Al2O3, Zr2O3, Si3N4 or AlN or a metal such as Si, Ti, Ni, Cu, Au, Ag, Co, Zr, Mo or W. However, they are impractical. In the former (ceramic) case, the interposing layer for bonding the monocrystalline SiC layer to the polycrystalline SiC substrate is an insulator and electric conduction on the back surface is unavailable upon device fabrication. In the latter (metal) case, metal impurity is incidentally entrained into the device, causing degradations of device performance and reliability.
Under the circumstances, many proposals have hitherto been made to overcome these shortcomings. For example, JP 5051962 (Patent Document 1) discloses a method involving the steps of providing a source substrate in the form of a monocrystalline SiC substrate having a silicon oxide thin film in which hydrogen or similar ions are implanted, providing an intermediate support of polycrystalline aluminum nitride having silicon oxide deposited on its surface, bonding the source substrate and the intermediate support at their silicon oxide surfaces, transferring the monocrystalline SiC thin film to the polycrystalline aluminum nitride (intermediate support), then depositing polycrystalline SiC, and thereafter immersing in a HF bath to dissolve the silicon oxide surface for division. However, when an SiC composite substrate of large diameter is prepared by this method, there arises the problem that a substantial bowing occurs due to the difference in coefficient of thermal expansion between the deposited polycrystalline SiC layer and the aluminum nitride (intermediate support). In addition, another problem can arise that the high interfacial energy at the interface between heterogeneous materials causes to form structural defects, which propagate into the monocrystalline SiC layer to increase the defect density.
JP-A 2015-15401 (Patent Document 2) discloses a method for stacking a monocrystalline SiC layer on a polycrystalline SiC support substrate, which is difficult to flatten its surface, without forming an oxidized film at the bonding interface, the method involving the steps of modifying with a high-speed atom beam the surface of the polycrystalline SiC support substrate to be amorphous without forming an oxidized film, and simultaneously modifying the surface of monocrystalline SiC to be amorphous, and bringing them in close contact and thermally bonding them. This method, however, has the shortcoming that since not only the separation interface of monocrystalline SiC, but also the crystal interior are partially modified with the high-speed atom beam, the originally monocrystalline SiC is difficultly restored to monocrystalline SiC of satisfactory quality even by subsequent heat treatment. When monocrystalline SiC of such quality is used as device substrates, templates or the like, devices of better properties or SiC epitaxial films of quality are obtainable with difficulty.
In addition to these shortcomings, the above technology has the problem that in order to bond monocrystalline SiC and a polycrystalline SiC support substrate together, the bonded interface must be as smooth as demonstrated by a surface roughness (arithmetic average roughness Ra according to JIS B0601-2013) of 1 nm or less. Even after monocrystalline SiC surface is modified to be amorphous, SiC which is known as difficult-to-machine material next to diamond takes a very long time in the subsequent smoothening process such as by grinding, polishing or chemical mechanical polishing (CMP). Thus a cost increase is unavoidable, which becomes a substantial barrier against practical application.
Further, the monocrystalline SiC layer undergoes a volume change at the time of crystallinity recovery, which induces expansion of internal stresses and defects (dislocations) occurring from polycrystalline/monocrystalline interface, and gives rise to the problem that the amount of bowing increases as the substrate becomes of larger diameter. When the modified stratum of monocrystalline SiC layer is an amorphous stratum, recrystallization of that stratum entails uniform nucleation, and formation of twins is thus unavoidable. In addition, a problem arises that since the process from ion irradiation to bonding is a continuous process in vacuum, the cost of the system is high. The problem of increased system cost also arises because ion implantation to a deep range (high energy) is necessary depending on the roughness of the substrate.
JP-A 2014-216555 (Patent Document 3) discloses a method involving the steps of bonding a first layer of monocrystalline SiC containing point defects onto a support substrate, and heating the layer along with the support substrate for rearranging the atom arrangement to extinguish point defects and line defects, and shutting off the influence of the crystal face of the support substrate on the overlying layer. However, there is the problem that point defects in the first layer are converted to complex defects during the heat treatment, which invite formation of twins and stacking faults. Another problem is that the substrate manufacture process becomes complicated since multiple stages of ion implantation are necessary in order to distribute point defects in the monocrystalline SiC layer (first layer). A further problem arises that dislocations occur depending on the energy level at the bonded layer interface with the support substrate.
JP-A 2014-22711 (Patent Document 4) discloses a method involving the steps of bonding an SiC layer having a low impurity density and low density defects onto a support substrate having a high impurity density and high density defects, and epitaxially growing thereon a layer having an impurity concentration necessary for a semiconductor device function, for thereby obtaining a low defect density layer equivalent to the low concentration layer. However, there remain outstanding problems that metal contamination occurs at the bonded interface and dislocations propagate from the high density substrate to the front layer side because crystal lattices are continuous from the substrate to the front surface.
JP-A 2014-11301 (Patent Document 5) discloses a method involving the steps of heating a support substrate of SiC to convert its surface to a carbon base layer, bonding a monocrystalline semiconductor layer to the surface, and thereafter causing cleavage over the entire area or in part. However, the method cannot be a means for obtaining a stable substrate because the layer containing SiC and carbon is bonded by fragile valence bonds so that the bonded interface is mechanically weak, and the carbon layer is damaged even in an oxidizing atmosphere.
JP-A H10-335617 (Patent Document 6) discloses a method for obtaining a semiconductor thin film on an insulating film, involving the steps of forming a hydrogen-occluded layer and an amorphous layer on a monocrystalline semiconductor substrate without resorting to ion implantation, bonding the amorphous layer to a support substrate, allowing for solid phase re-growth, and separating the monocrystalline semiconductor substrate. The method, however, has a possibility of twin formation during solid phase growth of the amorphous layer. Besides, there is the problem that the insulating film interposed prohibits the manufacture of substrates for discrete devices adapted for current flow in vertical direction.
The following document is also incorporated by reference as pertaining to the present invention. “Reduction of Bowing in GaN-on-Sapphire and GaN-on-Silicon Substrates by Stress Implantation by Internally Focused Laser Processing,” Japan Journal of Applied Physics, Vol. 51 (2012) 016504 (Non-Patent Document 1).